Process for producing ingots by electric resistance melting particulate metal under slag

ABSTRACT

A process for producing metal alloys in the form of large-size ingots, by melting under electro-conductive slag, wherein the material forming the alloy is partly supplied as solid metal and partly as metal in powder, granules or pellets, for conciseness called &#39;&#39;&#39;&#39;particulate metal&#39;&#39;&#39;&#39; comprising the sequential operations of: A. INTRODUCING INTO A MOLD, THE BOTTOM OF WHICH IS MADE OF ELECTRO-CONDUCTIVE MATERIAL, A CONSUMABLE-ELECTRODE UNIT, UNTIL CONTACTS THE SLAG; B. PARTIALLY FILLING WITH SAID PARTICULATE METAL A ZONE OR RECESS PROVIDED INSIDE SAID ELECTRODE UNIT AT ITS END CONTACTING THE SLAG, CLOSED AT ITS BOTTOM END BY A BOTTOM WALL, AND AT ITS TOP BY A FEEDING PLATE FOR THE POWDER METAL; C. ESTABLISHING IN SAID ZONE A CONTROLLED ATMOSPHERE BY A GAS CIRCULATION; D. INCREASING THE TEMPERATURE UNTIL OBTAINING THE GRADUAL MELTING OF SAID BOTTOM WALL AND THE GRADUAL SIMULTANEOUS SINTERING OF THE PARTICULATE METAL IN SAID ZONE UNTIL FORMING A SINTERED LAYER HAVING A THICKNESS SUFFICIENT TO SUPPORT THE WEIGHT OF THE SUPERPOSED PARTICULATE METAL; E. MELTING THE SO OBTAINED COMPOSITE ELECTRODE COMPRISING BOTH THE ELECTRODIC UNIT AND THE PARTICULATE METAL CONTINUOUSLY FED TO THE MELTING ZONE.

United States Patent Bianchi et al.

[451 Sept. 16, 1975 PROCESS FOR PRODUCING INGOTS BY ELECTRIC RESISTANCEMELTING PARTICULATE lVlETAL UNDER SLAG [75] Inventors: Paolo Bianchi;Aldo Ramacciotti;

Eugenio Repetto, all of Rome, Italy [73] Assignee: Centro SperimentaleMetallurgico S.p.A., Rome, Italy [22] Filed: Nov. 27, 1973 [21] Appl.No.: 419,380

[30] Foreign Application Priority Data Dec. 6, 1972 Italy 54535/72 [52]US. Cl 75/10 C; 13/9; 75/10 R;

[51] Int. Cl...... C22d 7/00; l-IOSb 7/18; B22d 27/02 [58] Field ofSearch 75/l012, 84;

[56] References Cited UNITED STATES PATENTS 2,541,764 2/1951 Herres75/10 R 2,782,245 2/1957 Preston 164/252 2,813,921 11/1957 Vordahl 75/10R 3,234,608 2/1966 Peras 75/10 R Primary Examiner-Peter D. RosenbergAttorney, Agent, or FirmY0ung 8L Thompson [57] ABSTRACT A process forproducing metal alloys in the form of large-size ingots, by meltingunder electro-conductive slag, wherein the material forming the alloy ispartly supplied as solid metal and partly as metal in powder, granulesor pellets, for conciseness called particulate metal comprising thesequential operations of:

a. introducing into a mold, the bottom of which is made ofelectro-conductive material, a consumable-electrode unit, until contactsthe slag;

b. partially filling with said particulate metal a zone or recessprovided inside said electrode unit at its end contacting the slag,closed at its bottom end by a bottom wall, and at its top by a feedingplate for the powder metal;

c. establishing in said zone a controlled atmosphere by a gascirculation;

(1. increasing the temperature until obtaining the gradual melting ofsaid bottom wall and the gradual simultaneous sintering of theparticulate metal in said zone until forming a sintered layer having athickness sufficient to support the weight of the superposed particulatemetal;

e. melting the so obtained composite electrode comprising both theelectrodic unit and the particulate metal continuously fed to themelting zone.

10 Claims, 1 Drawing Figure PATENTEU SEP 1 8l975 15%;. 9 05 O3 t euarlyt hemetals antlthe. Qthe ve em nts formingthe al oy ein supplied themelt ng zon more or less cont uouslyihthefor t uit blyi mixe 'par iel ofalloysno o met ording o, :oneo these pro-1 ces a etal: u ed whi h -i.s1' conti u y shaped to.ai na hdi meter t be, imme iately ebbVe the ingot,rnoldwhere the melting-process is being care b t; .thistube b side seringv the function o sunplying the metal to the alloy,t;s e rves,alsevthe function-of le trod and bf e nveybr e ce for the metal P 99 UHQdPQQdvlUIQ the. t be a t p, moves fre ly th b t bbut the leng therfyto op tinuously in the hottest zone of the liquid metallpool,anapresis l unde ithe end e ithe ele rod h main ad an a e .h thebretielly can; be o a n y th process consist inthat, ,it, is possible tocontrol at each moment the composition of the produced ingot by he i lfih f eQin J te fq pb b h P cl and l slsctretle; alsb; produ P t cul yphysically-and ehemieally hem gen eus is Obtai at least theoreticall lhe a etual drawbaeks a e due: to t e i of the t paratus for forming thestrip into a tube, which does not allow .the simultaneous utilization.ofmore than .two or threeelectrodes, thus prnti,ng,. the suitably rapidmanufacture 9f h s insets and. to he ratherfrequent ocean??? QfIQ Qggingof the leet ode: du t y the particles, dueto thesm all diametricaldimensions of the tube and to the slowness of theprocess. These W v c II ,in faetwifthez'electrode tube isclogged bytthepar cles itis;impossible to continuously adjust t he con positron of the, producedingot,

and moreover du tothe effect of the sudden change ness of the process,itis impossible to obviate immediately the formation of clogs, as theireffects are not immediately detectable, withthe consequence that thezoneshowing adifferentanalysis from'the optimum will greatly enlarge.

It is to benoted, moreover, that by the process known under the,ESRnameit has been impossible, hitherto, to obtainingots weighing morethan a few tons, if metal powders areuscd, [and weighing some tens oftons if an ingot is used as electrode. However, in certain sectors ofthe industry, for instance in the manufacture of rotors for bigalternators or turbines, particularly felt is the need of havingavailable remarkably heavy ingots, of upitosome' hundreds of tons, withan extremely closely controlled composition, and free fromholes,inclusions,segregations, etc. Such a kind of ingot, as regards thecharacteristics ofphysical and chemical, homogeneity, can be hardlyobtained .by the conventional methods of melting in a furnace andteeming into an ingot mold; however, it might be obtained with an ESRprocess if the technical limitations concerning the weight of the soobtained ingots were not present. t Y

A first attempt to solve this problem and' to meet avery=importantrequirement has been made by proposing a process includingsequentially the operative stages both of the conventional method ofingot casting and of the ESR process. This method consists inconventionallycasting an ingot having the required weight, in removingthe entire inner part along the longitudinal axis which is thatgenerally higher in physical and chemical defects, and in filling the soproduced recess, by a process according to the ESR techniques. Thisproposal appears not to have been accepted as the ingot obtainedaccording to this teaching has an outer zone which is not free from anyof the characteristic de fects of the big castings, namelystratifications of composition and physical defects, an inner zone withoptimum chemical and physical characteristics, and an intermediate zonebetween the other two zones having intermediate physical and chemicalcharacteristics. These differences of composition and of physicalcharacteristics in the radial direction and the presence in theoutermost layer, the most stressed, of most of the physical and chemicaldefects have brought the art to consider such an ingot unsuitable forthe manufacture of large-sized pieces (rotors for turbines, etc.).

The purpose of this invention is to eliminate the cited drawbacks,connected with the known ESR processes using metal particles, andallowing one to overcome the problem of the low limits of weight in theproduced ingots so as to allow ingots to be manufactured with uniformand controlled composition, free-from the'cited physical drawbacks andhaving a weight of some hundreds of tons.

According to the present invention a process is provided forthemanufacture of metal alloys in the form of large-size ingots, by meltingunder electroconductive slag, wherein the material forming the alloy issupplied inpart in the form of solid metal and in part underthe form ofparticulate metal, i.e., powder or grains or pellets, wherein with theterm particulate metal includes one or more metal elements or alloys,suitably mixed, said process being characterized by the combination ofthe following steps:

a. introducing into a mold having its bottom made of electro-conductivematerial a consumable electrode unit until it contacts the slag,

b. filling partially with said particulate material a zone locatedinside said electrode unit at the tip thereof contacting the slag, saidzone being delimited at its bottom by a bottom wall closing said end andat its top by a feeding plate for the metal particles;

c. establishing in said zone a controlled atmosphere by gas circulation;

d. increasing the temperature until obtaining the gradual melting ofsaid bottom wall and the gradual simultaneous sintering of the metalparticles in said zone until forming a layer having a thicknesssufficient to support the weight of the superposed column of theparticulate material;

e. melting the composite electrode so obtained, comprising both theelectrode unit and the metal particles continuously fed to the meltingzone.

Theiequipmentwused: according to this invention: for darryingJout thisprocess-includes "a consumable:elect-rode syste mwhichischaracterizedgby one or more cy-. lindricalwmetaltubes,preferablyone to four, for feedingxthe metal, each with adiameterivariable from 20 to 90 percent of the diameter of the ingot tobe produced, saidrtubesbeing:extendableby means of welding offurtherlportionstof tube to that, or to those, which is, or are: beingmeltedsthroughout the duration ofthe meltiQgprOCS.. r t r ilhe presentinvention \will be now described with particular :reference? to theattached drawing' showing by wayqf non lirnitative example one preferredem bodimentof theequipment according to thisinvention, usingasingleelectrode. I l h Rarticularlypin theattached drawing the FIGUREshows a sectional vertical view, of a scheme of a facility for themanufactureof ingots by the process according tothis'invention, 1

--With'=.reference to the FIGURE, therein are shown a mold 1, ofconventional ESR type, inside ofwhich has been shown theliquid metalpool, andthe superposed layer 6of molten slag, into which is immersedthe lower tipofa metaltubular'electrode 2 supported and guided bytechnologically suitable means, of a substantially conventional kind.altis tobe noted that said tubular electrode 2 isprovided in its lowerpart with a recess orzone 5 downwarddelimited, at the start of there-melting; by 'abottom wall 3 at least 35mm thick; closing the lowerend ofthe electrode 2, and at its top, by a feeding plat efl which willbe described in greater detail hereinafter;

readysintered metal particles under the" superposed column ofmetalparticles which drops downwards in the recess 5,passingthrough the duct8 and the hole or holes 11 of the plate 4.

As far as the course of the process according to this invention isconcerned, initially the lower -tipofthe electrode 2 submerged into theslag is closedby ametal bottom'wall 3 fixed to the walls ofthe'electro'de2 in side"of which isintro'duced the metal particlesoy afeeding plate 4. Thispla'te and the cited bottomwallB define a recess 5which is thus partially filled withthe metal particles. lnside saidrecess a controlled atmosphere will be established by a gas circulation,for instance using an inert gas such as argon. Then through eachelectrode an alternating or direct current (20 to 100 'V and 5,000to60,000 A) is caused to pass through the circuit the electrodes ofwhich arethe metal tube and the mold. This current, due to the Jouleeffect, develops a large amount of heat within the slag6under theelectrode 2, causing thus the gradual melting of the bottom wall 3 andsimultaneously the gradual sintering of. an ever increasing layer 7 ofmetal particles, inside said recess 5. When the bottom wall is entirelymelted, the sintered layer7 has still a thickness sufficient to supportthe superposed particle column 10, which is still lflCOl'lI'BnL Saidlayer 7 serves now the function of bottom wall and the melting processof the sintered layer and of simultaneous sintering of a layer of metalparticles assumes a stable operative condition.

The metal tube 2 serving the function of an electrode will be downwardsmoved at a rate of speed comprised between 1 and 10 cm per min., and theparticulate metal is fed so as to keep almost constant its volume withinthe recess 5.

' The feedingpIateA eonsis ts of aldisk madeleither of metal ororahyaarh'e'rfsuiraule materiaLhaS/inga diameter a little less thantheinternaldiameterof the electr0de2; said plate istransected by one ormore-holes l lg-provided with-delivery}valvesg into whichareinsertedgthe duct and the feeding: components 8 for the metal"particulesf and by one or' morewholes l2 into which are inserted theduct or ducts 9-for feedin g the ga'sfor establishing the controlledatmosphereambient. Thisplateds supported by" means different from saidfeeding ducts for the powder andfor the gas; and is located in fixedposition with respect to the mouth of the mold 1, or insidethe molditself, and in] thisfcase it: will be slow ly'upvvards movedin'ord'eflto-rhaintain constant its distance from 'themolten metal pool.The=feeding ducts forthe pa rtieules and-for the gas are independentfrom the electrode, said ducts 'can be disconnected from the plateandremoved from the electrode inlorder to; 'allo'w the coupling of theoperating electrode tothe subsequent lengths-oftube. 1 1 t 9 The metalwhich willbd'sup'plid'insidethe cylindrical-"t'ube 2 and whichfillsthe'recess 5can be supplied, as afores'aid,=in the form ofpowde r;ofgranules or of pelletsi"* :1: i i lf'only powders ar'e used, themostsuitable grain sizes are the finest available onwhe-mar'ketyand thelower limit of sizeis' determined only bythe cost of the 'pow ders themselvesfif I y Itis possible howeveri 'to'use coarse powders, withaLmaximum grainsize of 4mrn,iand intliiscaseit isnecessary to mixsaid'coarse powders with finer powders in the percent of at least 20percent byweight soasto obtain for the mixture an apparent 'bulk densityof at least- 2.7"grains/cc. i Q" Ontheotherhand, if granules orpellet'sare tobe used itwill be necessary/to mix therewith at least20percent b'yfweight'of a mixture of powders and srnall gr'anules, thesi'ze of wh ichis between 0:01 and rmm, soastoob tainafinal mixturehaving an apparent bulk density of at-least 2.6 grams/cc Y in thelightof the practical-and theoretical inrdrma: tionavailable before theexperiments which ledto this invention, thesol'uti on' here proposedforthe technical problemof the production orbig 'ingbtsivith the ESRteafii ueabneard to be doubtfully applicableflnfact, aset ofconsiderations as to the sintering rates of the non' compressedmetalparticles, as tdtherhelting rate whichrn ustberather high in ordertoallowbig ingots to bejmariufactured with industrially acceptable speed,asf't othe" mechanical resistance of thefisirrtered metal prodiietsathighternperatures, and as to the thermal arid el ectrical conductivityof the metal powders in incoherent mass, caused the art to consider theprocess as above describ'ed to be hardly capable o fbeing practiced.

Thesubsequent tests, on the contrary, demonstrated, that other-theoretically more effective solutions were r M, -\;1M., ,i not capableof be ngpractlced foreither economical or composition by weight asfollows:

slag havingthe i CaF 40%; Al O 30%; CaO 24%; MgO 6% C 0.25; Mn 0.50; P0.01; S 0.009; Si 0.25; Ni 3; Cr 0.30; O 60 ppm the balance consistingof Fe and minor impurities.

The used alternating current at 50 H had 50 V and 20,000 A.

In steady condition of operation, the temperature of the slag was 16501,700C; the downwards feed rate of the electrode was about 3.3 cm/min.,and the supply rate of the powder was 500 kgs/h.

After 24 hours an ingot has been obtained weighting tons and having thecomposition as follows:

C 0.27; Mn 0.46; P 0.008; Si 0.13; Ni 30; Cr 0.30; S 0.008; 0 50 ppm thebalance consisting of Fe and minor impurities.

This composition appeared to be substantially constant throughout thelongitudinal and transverse sections of the ingot and also themechanical properties appeared to be uniform.

The present invention has been described with particular reference tocertain specific embodiments thereof, but it is to be understood thatmodifications might be entered without thereby departing from the scopeof the invention as defined by the appended claims.

Having thus described the present invention, what is claimed is:

1. An ESR process for producing metal alloy ingots by melting underelectro-conductive slag, comprising establishing in anelectro-conductive mold a quantity of slag, introducing into said mold ahollow consumable metal electrode having a closed bottom immersed insaid slag, introducing particulate metal into said hollow electrode,establishing an inert gas atmosphere in said hollow electrode, passingan electric current through said electrode and mold to melt said closedend of the electrode and to sinter said particulate metal adjacent saidend of the electrode to form a sintered layer having a thicknesssufficient to support the weight of the su perposed particulate metal,and supplying further particulate metal to the interior of saidelectrode while continuing to pass said electric current therebyprogressively to melt said particulate metal and said electrode andprogressively to build a said ingot in said mold.

2. A process as claimed in claim 1, and feeding said electrodedownwardly into said mold at a rate between 1 to 10 cm/min.

3. A process as claimed in claim 2, and welding further hollowelectrodes to said hollow electrode to replace the material of theelectrode consumed by melting.

4. A process as claimed in claim 1, in which said particulate materialhas a grain size up to 4 mm.

5. A process as claimed in claim 1, in which the bulk density of saidparticulate metal is at least 2.7 g/cc.

6. A process as claimed in claim 1, in which said elec tric current is20 to 100 volts and 5,000 to 60,000 amperes.

7. A process as claimed in claim 1, and maintaining the end of saidelectrode a constant distance above the level of the metal bath thatforms atop said ingot.

8. A process as claimed in claim 1, in which said electrode has adiameter from 20 percent to percent the diameter of the ingot to beproduced.

9. A process as claimed in claim 1, said particulate metal and electrodebeing steel.

10. A process as claimed in claim 1, said particulate metal and saidelectrode having the same composition. i

1. AN ESR PROCESS FOR PRODUCING METAL ALLOY INGOTS BY MELTING UNDERELECTRO-CONDUCTIVE SLAG, COMPRISING ESTABLISHING IN ANELECTRO-CONDUCTIVE MOLD A QUANTITY OF SLAG, INTROUDCING INTO SAID MOLD AHOLLOW COMSUMABLE METAL ELECTRODE HAVING A CLOSED BOTTOM IMMERSED INSAID SLAG, INTRODUCING PARTICULATE METAL INTO SAID HOLLOW ELECTRODE,ESTABLISHAS INERT GAS ATMOSPHERE IN SAID HOLLOW ELECTRODE, PASSING ANELECTRIC CURRENT THROUGH SAID ELECTRODE AND MOLD TO MELT SAID CLOSED ENDOF THE ELECTRODE AND TO SINTER SAID PARTICULATE METAL ADJACENT SAID ENDOF THE ELECTRODE TO FROM A SINTERED LAYER HAVING A THICKNESS SUFFICIENTTO SUPPORT THE WEIGHT OF THE SUPERPOSED PARTICULATE METAL, AND SUPPLYINGFURTHER PARTICULATE METAL TO THE INTERIOR OF SAID ELECTODE WHILECONTINUING TO PASS SAID ELECTRIC CURRENT THEREBY PROGRESSIVELY TO MELTSAID PARTICULATE METAL AND SAID ELECTRODE AND PROGRESSIVELY TO BUILD ASAID INGOT IN SAID MOLD.
 2. A process as claimed in claim 1, and feedingsaid electrode downwardly into said mold at a rate between 1 to 10cm/min.
 3. A process as claimed in claim 2, and welding further hollowelectrodes to said hollow electrode to replace the material of theelectrode consumed by melting.
 4. A process as claimed in claim 1, inwhich said particulate material has a grain size up to 4 mm.
 5. Aprocess as claimed in claim 1, in which the bulk density of saidparticulate metal is at least 2.7 g/cc.
 6. A process as claimed in claim1, in which said electric current is 20 to 100 volts and 5,000 to 60,000amperes.
 7. A process as claimed in claim 1, and maintaining the end ofsaid electrode a constant distance above the level of the metal baththat forms atop said ingot.
 8. A process as claimed in claim 1, in whichsaid electrode has a diameter from 20 percent to 90 percent the diameterof the ingot to be produced.
 9. A process as claimed in claim 1, saidparticulate metal and electrode being steel.
 10. A process as claimed inclaim 1, said particulate metal and said electrode having the samecomposition.